The present invention relates to plate-like barium ferrite particles for magnetic recording, comprising plate-like particles represented by the formula: EQU BaFe.sub.12-2x M.sub.x Ti.sub.x O.sub.19
wherein x is a number of from 0 to 1.2 and M represents Co(II) or Co(II) and a divalent metal ion other than Co(II), the surface of the plate-like particles being modified with a magnetite represented by the formula: EQU FeO.sub.y.Fe.sub.2 O.sub.3
wherein y is a number of over 0 and not more than 1, and a process for producing the plate-like barium ferrite particles.
In recent years, accompanying with the propagation of VTR, audio component, word processor and computer, ferromagnetic non-acicular particles of a favorable dispersibility provided with a suitable coercive force(Hc) and a large value of magnetization (M) are demanded as a magnetic material for recording, particularly as the magnetic material for perpendicular magnetic recording.
In general, as the ferromagnetic non-acicular particles, barium ferrite particles have been well known. However, the coercive force of the barium ferrite particles obtained by the dry process is ordinarily higher than 3000 Oe, and because of such a high coercive force, barium ferrite particles are not favorable as a magnetic material for magnetic recording, namely, as the magnetic material for use in magnetic recording, barium ferrite particles having a coercive force of from 300 to 1000 Oe are demanded.
As a method for reducing the coercive force of barium ferrite particles, for instance, a method has been already known wherein a part of Fe(III) in the barium ferrite is substituted with other metal ion (for instance, Co(II) and Ti(IV)), thereby reducing the coercive force wherein the change of saturation magnetization thereof is minimized as far as possible.
Further, since the conventional barium ferrite particles obtained by sintering are composed of polycrystals of an average diameter of a few micrometers and the average diameter of the particles is at least one micrometer even after pulverization thereof, such particles are poor in dispersibility in paints and are not favorable as the magnetic particles for magnetic recording. In short, as the magnetic particles for use in magnetic recording, those which are as fine as possible, particularly those of an average particle diameter of 0.05 to 0.3 .mu.m have been demanded. This fact is shown in many published literatures, for instance, in Japanese Patent Application Laid-Open No. 53-20596 (1978) ". . . in the case where the average particle diameter is over 0.5 .mu.m, there is a difficulty in uniformly dispersing such particles in paints. . . . . ", in Japanese Patent Application Laid-Open No. 56-125219 (1981) "In the range of recording wavelength of shorter than 1 .mu.m, the perpendicular magnetic recording is more useful as compared with longitudinal magnetic recording, and in order to carry out the sufficient recording and regeneration in such a wavelength region, it is preferable that the particle diameter of the ferrite is less than about 0.3 .mu.m. However, since the particles of about 0.01 .mu.m in diameter do not exhibit the desired ferromagnetism, as an preferable diameter of particle, the range of 0.01 to 0.3 .mu.m is demanded." and in Japanese Patent Application Laid-Open No. 57-212623 (1982) "it is suitable that the average diameter of magnetic barium ferrite particles is less than 0.3 .mu.m, and particularly, those fine particles of 0.03 to 0.3 .mu.m in average particle diameter are favorably suitable. Because, the particles of below 0.03 .mu.m in average particle diameter do not exhibit the ferromagnetism sufficient for magnetic recording, and the particles of over 0.3 .mu.m in average particle diameter can not advantageously carry out the magnetic recording as the high density recording."
Further, it is necessary that the magnetization of the magnetic particles is as large as possible, and the fact is disclosed, for instance, in Japanese Patent Application Laid-Open No. 56-149328 (1981). "It is required that the saturation magnetization of magnetoplumbite ferrite used as the material for medium of magnetic recording is as large as possible."
On the other hand, as one of the processes for producing barium ferrite, a process of thermally treating an aqueous alkaline suspension containing Ba-ions and Fe(III) in an autoclave as the reactor (the process is hereinafter simply referred to as "the autoclaving process") has been hitherto known. According to the autoclaving process, barium ferrite particles precipitate under the suitably selected reaction conditions. The thus precipitated particles are hexagonal plate-like particles, the distribution of the particle size and the average size of the particles differ according to the reaction conditions resulting in the difference of the magnetic properties of the particles.
The present inventors who have engaged in studies and development of the plate-like barium ferrite particles obtained by the autoclaving process has known that the plate-like barium ferrite particles of 0.05 to 0.3 .mu.m in average particle diameter are available depending on the reaction conditions in the autoclaving process, however, in the case where the platelike barium ferrite particles of 0.05 to 0.3 .mu.m in average diameter and of a large magnetization, particularly of 40 emu/g are produced, such a obtained particles show a coercive force of larger than 1000 Oe.
Namely, the present inventors have conducted various examinations for obtaining the plate-like ferrite particles suitable for magnetic recording, showing a coercive force of 300 to 1000 Oe and an average particle diameter of 0.05 to 0.3 .mu.m. As a result of the examination, the present inventors have known that, by the autoclaving process, in the case where the plate-like barium ferrite particles of BaFe .sub.12-2x M.sub.x Ti.sub.x O.sub.19 wherein x is 0 to 1.2 and M represents Co(II) or Co(II) and other divalent metal ion than Co(II) are formed in the presence of Ti(IV) and Co(II), or in the presence of Ti(IV), Co(II) and a divalent metal ion other than Co(II) such as Fe, Zn and Mn, it is possible to obtain the plate-like barium ferrite particles of a coercive force of 300 to 1000 Oe and an average particle diameter of 0.05 to 0.3 .mu.m (refer to European Patent Application Laid-Open No. 0123445).
The coercive force (Hc) of the thus formed plate-like barium ferrite particles of BaFe.sub.12-2x M.sub.x Ti.sub.x O.sub.19 by adding Co ions as M(II) lowers in the case where x becomes larger, and the coercive force thereof is 100 Oe when x is 1.2.
However, the magnetization (M) in the magnetic field of 10 KOe of the thus obtained barium ferrite particles was only less than 40 emu/g in the case wherein x was higher than 0.3.
By the way, in the case where a part of Co(II) as M(II) was substituted by Fe, Zn or Mn, the magnetic properties show some change depending on the kinds and/or the amount of the metal ion.
In consideration of the foregoings, as a result of variously examining the process for further raising the magnetization of the plate-like barium ferrite particles of BaFe.sub.12-2x M.sub.x Ti.sub.x O.sub.19, wherein x is 0 to 1.2 and M represents Co(II) or Co(II) and other divalent metal ion than Co(II), showing a coercive force of 300 to 1000 Oe and 0.05 to 0.3 .mu.m in average particle diameter, obtained by the autoclaving process, the present inventors have found that
(1) in the case where the plate-like barium ferrite particles of BaFe.sub.12-2x M.sub.x Ti.sub.x O.sub.19 wherein 0.ltoreq..times..ltoreq.1.2 and M represents Co(II) or Co(II) and divalent metal ion other than Co(II), obtained by the autoclaving process, are mixed with an aqueous suspension of Fe(OH).sub.2 of pH 8.0 to 14.0 containing Fe(II) in an amount of not more than 50 atomic % to all Fe(III) and the resultant mixture is thermally treated at 50.degree. to 100.degree. C. in a non-oxidative atmosphere, whereby the surface of the particles is modified with a magnetite of FeO.sub.y. Fe.sub.2 O.sub.3 wherein 0&lt;y.ltoreq.1, the magnetization of the thus modified particles can be raised,
(2) the modifying rate of the magnetite depends on the concentration of excessive sodium hydroxide of the aqueous suspension and the temperature of thermal treatment, and
(3) the magnetic properties and electric conductivity of the thus obtained particles by modifying the particles of a composition of BaFe.sub.12-2x M.sub.x Ti.sub.x O.sub.19 wherein x and M are as defined above with magnetite of FeO.sub.y.Fe.sub.2 O.sub.3 wherein y is as defined above depend on the composition thereof and the modifying rate of the magnetite, and based on the findings (1), (2) and (3), the present invention has been attained.